Internal combustion engines are using several current controlled solenoid valves. Normally, a solenoid valve is electrically actuated by the ECU via a pulse width modulated signal (PWM), expressed as a duty cycle (DC) in percentage. As known, pulse width modulated (PWM) is a modulated technique that conforms the width of the pulse, formally the pulse duration, based on a modulator signal information. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is. The term duty cycle describes the proportion of ‘on’ time to the regular interval or ‘period’ of time; a low duty cycle corresponds to low power, because the power is off for most of the time. As mentioned, duty cycle is normally expressed in percent, 100% being fully on.
Many modern engines are provided with a fuel injection system for directly injecting the fuel into the cylinders of the engine. The fuel injection system generally comprises a fuel common rail and a plurality of electrically controlled fuel injectors, which are individually located in a respective cylinder of the engine and which are fluidly connected to the fuel rail through dedicated injection lines. Each fuel injector generally comprises a nozzle and a movable needle which repeatedly opens and closes this nozzle, and fuel can thus be injected into the cylinder giving rise to single or multi-injection patterns at each engine cycle.
The needle is moved with the aid of a dedicated actuator, typically a solenoid valve, which is controlled by the ECU. The ECU operates each fuel injection by generating an electric command, via a pulse width modulated signal (PWM), causing the actuator to open the fuel injector nozzle for a predetermined amount of time, and a subsequent end of command (EOC), causing the actuator to close the fuel injector nozzle. The time between the electric opening command and the EOC is generally referred as energizing time (ET) of the fuel injector, and it is determined by the ECU as a function of a desired quantity of fuel to be injected.
For a solenoid valve, which is driven via a PWM signal, the electrical control of the current shape is affected by errors. In particular, the error introduced by the uncontrolled starting point of the final current switch-off before the final EOC current switch-off. This error can be considered mainly a slowly variable delay that influences the accuracy of the solenoid valve control. The error is also present in commands with the same energizing time.
FIG. 3 shows an example of the mentioned error affecting an injector current control of two pulses with the same energizing time. The graph is a plot of the solenoid valve current versus time. As can be seen from the figure, the two current pulses 500, 510 having the same energizing time, have the end of command EOC exactly at the same time. Notwithstanding this, the current shape of the pulses differently behaves and at a given current value (for example a current value of 1.4 A, which is still able to maintain the injector needle open, i.e. to let the fuel injection continue) current pulse 510 has a time delay Δti of about 2 μs. This uncontrolled error causes a remarkable variation in the fuel injection quantity (above all, in case of small injection quantities), since a common rail injector has a typical fuel quantity vs. command time sensitivity of about 0.15-0.3 mm3/μs at 200 MPa. This error is due to the operating conditions: for example, environment temperature and/or system voltage, influence the PWM signal and consequently the end of the injection.
Therefore a need exists for a method of controlling a solenoid valve, which does not suffer of the above inconvenience.
In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.